1. Field of the Invention
The present invention relates to a method and device for forecasting/detecting a polishing end point and a method and device for monitoring a real-time film thickness, in particular, it relates to a method and device for forecasting/detecting a polishing end point and a method and device for monitoring a real-time film thickness, for precisely forecasting/detecting a polishing end point, while suppressing Joule heat loss due to an eddy current in a chemical mechanical polishing (CMP) at the minimum, and precisely evaluating in real-time whether a predetermined conductive film has been appropriately removed.
2. Description of the Related Art
There has been know a process is known in which, for example, an oxide film is formed on a surface of a semiconductor wafer, and lithography and etching are performed on the oxide firm and a groove pattern corresponding to a wiring pattern is formed, and a conductive film that is made of Cu and the like to fill up the groove pattern is formed thereon, and other unnecessary portion other than the filled up portion such as the groove pattern or a through hall part and the like of the conductive film are removed by the chemical mechanical polishing, thereby forming a wiring pattern. In this wiring pattern formation, it is extremely important to precisely detect the polishing end point at the moment when the conductive film of the unnecessary parts is removed at appropriate thickness and stop the process. If the polishing of the conductive film is excessive, the resistance of the wiring increases; meanwhile, if the polishing is insufficient, insulation failures of the wiring occur.
As a conventional technology related to this, for example, the following monitor method on the spot of a change of film thickness is known. This conventional technology is a method to monitor the thickness change of the conductive film in a method to remove a conductive film by chemical mechanical polishing from a substrate main body (semiconductor wafer) on the spot, and a sensor including a serial or parallel resonance circuit of an inductor consisting of a coil wound around a ferrite pot type core for shaping an electromagnetic field so as to have directivity to the same and a capacitor is arranged at the vicinity of the conductive film, and a sweep output consisting of a frequency of 20 Hz to 40.1 MHz from an excitation signal source is applied to the sensor through impedance means for movement point setting. Thereby, when the sensor is excited, an oscillation electric current flows into the coil and, an alternating electromagnetic field occurs. Subsequently, this alternating electromagnetic field guides an eddy current into the conductive film. When the eddy current is guided to the conductive film, two effects occur. Firstly, the conductive film acts as loss resistance, and the effect is a resistance load to the sensor circuit, and this lowers the amplitude of the resonance signal, and lowers the resonance frequency. Secondly, when the thickness of the conductive film decreases, an effect in which a metal rod is pulled out from the coil of the inductor occurs, and causes a change of the inductance and a frequency shift. By monitoring the change of the frequency shift related to the sensor resonance peak due to the thickness change of the conductive film in this manner, the thickness change of the conductive film is continually detected (for example, see Patent Document 1).
Further, as another conventional technology, for example, the following eddy current sensor is known. This conventional technology is equipped with a sensor coil (an eddy current sensor) arranged at the vicinity of a conductive film or a substrate on which a conductive film is formed, an alternate current signal source that supplies an alternate current signal of a constant frequency around 8 to 32 MHz to the sensor coil and forms an eddy current in the conductive film, and a detection circuit that measures reactance components and resistance components including the conductive film, and the sensor coil is equipped with an oscillation coil connected to the signal source, a detection coil arranged at the conductive film side of the coil, and a balance coil arranged at the opposite to the conductive film side of the oscillation coil, and the detection coil and the balance coil are connected so as to mutually reverse phase. Then, synthetic impedance is output from the resistance components and the reactance components detected by the detection circuit, and the changes of the film thickness of the conductive film are detected as approximately linear relations in a wide range from the changes of the impedance. (For example, see Patent Document 2.)
Furthermore, as still another conventional technology, for example, the following eddy current sensor is known. In this conventional technology too, in the same manner as in the conventional technology shown above, in [0008], it is described that the magnetic flux that a sensor coil forms penetrates a conductive film on a substrate arranged on the entire surface of the sensor coil, and changes alternately and causes an eddy current in the conductive film, and the eddy current loss occurs because the eddy current flows into the conductive film, and decreases reactance components of the impedance of the sensor coil when viewed as an equivalent circuit. Further, in [0009], it is described that by observing changes of the oscillation frequency of the oscillation circuit, when a conductive film gradually becomes thin, with the progress of polishing, the oscillation frequency decreases by this, and becomes the self oscillation frequency of the tank circuit where the conductive film completely disappears by polishing, and after that, the oscillation frequency becomes roughly constant. Therefore, by detecting this point, the end point of the chemical mechanical polishing of the conductive film can be detected. Furthermore, in [0025], it is described that as shown in FIG. 2, the eddy current loss changes when the polishing of the conductive film progresses, and the equivalent resistance value of the sensor coil changes. Therefore, the oscillation frequency of the oscillation circuit changes, and by dividing this oscillation signal by a frequency dividing circuit, or subtracting the same by a substractor, a signal corresponding to the size of the frequency of the detection width is displayed on a monitor. Thereby, a transition of frequency locus as shown in FIG. 2 mentioned above is obtained (for example, see Patent Document 3).
[Patent Document 1] Japanese Patent Application Publication No. 2878178 (pages 2 to 7, FIGS. 1 to 15)
[Patent Document 2] Japanese Patent Application Publication No.3587822 (page 3, FIGS. 1 to 11).
[Patent Document 3] Japanese Patent Application Laid-Open No.2003-21501
In the conventional technology described in the Patent Document 1, a serial or parallel resonance circuit of an inductor consisting of a coil wound around a ferrite pot type core for shaping an electromagnetic field so as to have directivity to the same and a capacitor is arranged. Then, a sweep output consisting of a frequency of 20 Hz to 40.1 MHz is applied to the sensor at the early stage of polishing, and by an alternating electromagnetic field having the directivity occurred from the coil, a leakage magnetic flux that penetrates the conductive film is generated and a large eddy current corresponding to the film thickness of the conductive film is guided from the early stage of polishing. In order to guide a large eddy current corresponding to the film thickness of the conductive film, it is necessary to form a large alternate current electromagnetic field, that is, a large magnetic flux of such a degree as to penetrate the conductive film, and the monitor of the thickness changes of the conductive film is carried out by use of the eddy current guided into the conductive film from the early stage of polishing to the end stage of polishing. Therefore, during the monitor of the film thickness changes, it is necessary to make the magnetic flux penetrate toward the thickness direction of the conductive film. The above is clear from the fact that a magnetic flux line penetrating the conductive film is described in the part of all conductive films, in the drawings of the bulletin concerning the Patent Document 1.
On the surface of the wafer in the early stage of polishing, it is common that there is a pure Cu film (a conductive film) at the top layer. A very large leakage magnetic flux is necessary to guide an eddy current to all of these pure Cu films. However, the leakage magnetic flux induces eddy currents, but they are consumed as a Joule heat in form of the eddy current loss soon. Because this Joule heat loss has a small volume resistance to the pure Cu film at the most outer layer, the heat generation is comparatively small, but in the inside part which is already wired, because a wiring cross sectional area is small and its volume resistance is small, a large eddy current is induced by penetrating magnetic flux, and as a result, a large Joule heat loss is locally produced. This occasionally causes a problem that the wiring is partially molten and disconnected. It becomes a condition of so-called induction heating, and particularly become the phenomenon that the inside is full of heat. In particular, in a Cu wire and the like, when Cu is heated, Cu may diffuses into barrier films of Ta and the like, and in some cases, Cu breaks through barrier films and diffuses.
Further, when several layers of wires are arranged in the surface section of the wafer, furthermore to the fear of the Cu film of the surface layer, the internal wiring part whose processing has already been completed is warmed partially and spreads to circumference, and dopant forming p type, n type in the semiconductor substrate diffuses more, and there is a possibility that the characteristics of the element in the substrate are changed. Furthermore, even when heat does not occur, if an excessive eddy current flows in minute wires, there is a possibility that electromigration is induced and wires may be disconnected.
Furthermore, for example, at the moment of a predetermined remaining film amount near the polishing end point, when a process is made by changing polishing conditions, it is difficult to ascertain whether it is a predetermined remaining film amount or not. Changes from an initial film thickness can be guessed, but when the initial film thickness varies, the estimate of predetermined remaining film amount varies. As for the judgment on this polishing end point, when a gap between the sensor and the conductive film finely changes by vibration of the polishing, the floating capacity of the whole sensor circuit system changes, and the whole resonance frequency shifts. Therefore, the judgment on a polishing end point by the setting of the threshold value becomes difficult if a resonance frequency totally shifts even if the threshold value is set at the moment of a resonance frequency of a certain setting so as to make a setting to determine the polishing end point. Thus, in the conventional method, even if the threshold value is set to a certain value, in resonance frequency that increases or decreases monotonously and continuously, when the gap between the sensor and the conductive film changes finely, or there is something dielectric between them, the waveform itself moves upward and downward entirely, as a result, the preset threshold value set beforehand does not have any meaning frequently.
In the conventional technology described in the Patent Document 2 using an eddy current sensor too, the monitor of the film thickness changes of the conductive film is made by the changes of the eddy current from the early polishing stage to the end polishing stage, which is almost same as the conventional technology described in the Patent Document 1.
Further, in the conventional technology to monitor the film thickness of the conductive film by use of eddy currents from the early polishing stage to the end polishing stage, it is necessary to make a magnetic flux strong enough to penetrate into the film in order to induce eddy currents in the film, and as for the shape of the inductor, it is of a three-dimensional shape to be able to give directivity to the magnetic flux. Therefore, there is generally the following problem when to incorporate a sensor in a polishing device. An electric current that flows in the coil becomes large, and electricity consumption increases, and the power supply unit becomes large. Magnetic flux leaks out to the outskirts, and a noise is easy to occur. Processes to surround conducting wire in the shape of a coil are required and increase costs.
In the conventional technology consisting of the eddy current sensor described in the Patent Document 3, at first, as for hardware of this sensor part used in this conventional technology, at first, it is a structure assuming the sensor coil penetrating a conductive film. Therefore, in hardware where only a magnetic field of the degree that does not penetrate a conductive film occurs, eddy currents cannot be formed and the purpose cannot be achieved. Further, because the conductive film is decreased by polishing, the region where an eddy current is formed decreases monotonously, and therefore the oscillation frequency decreases monotonously, and the moment when the oscillation frequency becomes roughly constant is considered as the end point and this point is detected. In the algorithm of this software to use in this conventional technology, as a change of the oscillation frequency, the change from decrease to rough uniformity is made the change of the oscillation frequency, and, for example, in the case when this oscillation frequency has an inflection point, the algorithm cannot be detected. Furthermore, magnetic flux penetrates a conductive film from the early stage of the polishing, and an eddy current is in regular condition to occur as shown in FIG. 2. Herein, the eddy current sensor generates an eddy current positively from beginning to end, and generally speaking, an eddy current sensor has a method of recalculating a film thickness change from the eddy current change.
Therefore, there are technological problem to be solved for precisely forecasting/detecting a polishing end point, and precisely calculating the remaining film amount to be removed and the polishing rate and the like, and precisely evaluating whether a predetermined conductive film has been appropriately removed, without giving strong magnetic flux to minute wires formed in a film, as a result, restraining the occurrence of an eddy current induced by magnetic induction, and suppressing Joule heat loss due to the eddy current at the minimum, and eliminating the situation in which the quantity of eddy current induced shifts entirely, due to the changes of the gap between the sensor and the conductive film and the intervening normal state of the dielectric substances such as slurry and the like, and the setting of the threshold value largely changes and detection becomes hard, and thereby it is possible to sufficiently precisely detect even a minute magnetic field of the degree that does not penetrate the device wafer, and the present invention is aimed at solving these problems.